Many cables, in particular cables for the transmission and/or distribution of power, may be susceptible to failure in a fire-related emergency. Many cables are not designed to sustain operation at high and/or rapidly increasing temperatures, as experienced in a fire.
The fire resistance of an electrical cable may be evaluated and certified by national and international standards. These standards generally involve testing the electrical cable to prove its capacity for operating in the presence not only of fire for a given time span, but also of water possibly coming from sprinklers or hoses.
Fire resistant (or resistive) cables may be evaluated for compliance with standards developed by the US certification company known as Underwriters Laboratories (UL), such as UL Standard 2196, 2012 (“UL-2196”). To obtain certification, cables are tested under fire conditions. During the test, the cables are installed in conduits, e.g., the tubing system used for protection and/or routing of the cable, and the conduits are mounted on a fire wall, e.g., a wall that restricts the spread of fire, either vertically or horizontally in accordance with the particular test. The conduits may contain multiple cables, and the cables may fill the respective conduit to no greater than 40% as according to NFPA (National Fire Protection Association) 70: National Electrical Code (NEC). The cables are tested at the maximum-rated voltage of the cable or the utilization voltage of the cable, and remain energized throughout the test. Temperature rise and fire conditions are prescribed. After the test, the cables are de-energized, and the wall is hosed down to determine the structural integrity of the installed system. After the hose stream, the cables are re-energized in 30 minutes or less to assess the electrical integrity of the cables.
The conduits that pass the test are certified in a given configuration. For example, if a conduit with a 14% conduit fill passes the test, but does not pass the test with a 32% conduit fill, then only the conduit with the 14% conduit fill is certified. However, a conduit passing the test with a higher fill also certifies a conduit having a lower fill.
Certification under UL-2196 may involve a one-hour test or a two-hour test. In 2012, research conducted by UL showed that some products and systems similar to those previously certified under UL-2196 could no longer consistently pass the two-hour fire wall test. UL initiated an interim program with more stringent revised guidelines for certification. Cables that have achieved certification under the interim program typically include metallic coverings or armor, but the provision of armor makes the cable heavier, more expensive, and less flexible.
One method of improving the high temperature performance of a cable includes providing the cable with an extruded covering formed of one or more heat resistant materials. The extruded coverings may incorporate fillers to increase heat resistance.
Another method of improving the high temperature performance of a cable includes providing the cable with mica tape made with glass fibers on one side of the mica tape and mica on the opposite side of the mica tape. The mica tape is wrapped around a conductor during production, and one or more outer layers are applied over the layer of mica tape. Upon being exposed to increasing temperatures, the outer layers may degrade and fall away, but the glass fibers may hold the mica in place.
Mica tape manufacturers typically instruct users to apply the mica tape with the mica side facing the conductor. For example, the brochure from Cogebi Inc. for Firox® P discloses a tape made of phlogopite mica paper bonded to an electrical grade glass cloth as the supporting fabric and impregnated with a high temperature resistant silicone elastomer. The brochure discloses that the tape is applied over a conductor with the mica side facing the conductor to act as electrical insulation in the event of fire.
Also, the brochure from Von Roll Switzerland Ltd for Cablosam® 366.21-30 discloses a flexible muscovite Samica® tape impregnated with a silicone resin and reinforced with woven glass. The woven glass forms a backing surface. The brochure discloses that the tapes are applied onto the bare wire strand always with the woven glass to the outside after application. Thus, the brochure describes that the tape is applied to the conductor with the mica side facing the conductor.
European Publication EP 1 798 737 (EP '737) discloses an electric cable including a plurality of electrically conductive wires, on each of which is applied a layer comprising a glass fiber strip with a mica layer glued thereon. EP '737 applies a single mica layer and does not disclose which side of the layer with the glass fiber strip and the mica layer faces the conductive wires.
PCT International Publication WO 96/02920 (WO '920) discloses a cable including two layers of glass-cloth-backed mica tape applied over a wire conductor. WO '920 discloses that the mica tapes layers are applied with the glass cloth on the outside of the layer, and therefore that the mica side faces the conductor.
European Publication EP 1 619 694 (EP '694) discloses a cable including a conductor on which two layers of tape including glass cloth adhesively coated on one side with mica is applied. EP '694 discloses that each layer is applied with the mica side facing the conductor.
French Publication FR 2 573 910 (FR '910) discloses an insulating layer for electric cables with dielectric and insulating characteristics over a large temperature range. This layer comprises one or more mica layers obtained by helicoidally wrapping one or more tapes made of a glass fabric impregnated by an adhesive supporting mica particles. The mica surface with mica particles is preferably provided facing the structure to be protected. The manufacturing process provides for helicoidally wrapping a first mica tape around the element to be protected by positioning the surface with mica particles to face the element to be protected; and a second mica tape is superposed on the first one with the face covered with mica particles inwardly turned, but with a rotation direction opposite to that of the first tape. All of the mica tapes used has the respective mica surfaces facing the conductors.
The Applicant faced the problem of providing a fire-resistant cable suitable for complying with national and international standards and experienced that the known cables, such that of FR '910, may not be able to pass some tests for obtaining fire-resistance certification.
Fire resistance might be improved by wrapping additional layers of mica tape around the conductor, but increasing the number of layers of mica tape may increase the weight and size of the cable, and may also increase the cost and time to manufacture the cable.